1. Field of the Invention
The present invention relates to moulding compositions with diffusing properties, which include matrix polymers and bead polymers with an average particle size in the range from 5 to 40 xcexcm, whose refractive index nD at 20xc2x0 C. differs from that of the matrix polymer, where the bead polymers are obtainable by a process in which a polymerizable mix which includes at least 50% by weight of (meth)acrylates is dispersed in an aqueous phase and polymerized. The present invention further relates to light-scattering mouldings which are obtainable by thermoplastic moulding of the moulding compositions of the invention.
2. Discussion of the Background
There are a number of reasons for giving standard moulding compositions light-scattering or diffusing properties, for example to produce protective covers for light fittings.
Materials typically used for this purpose include traditional opacifiers, e.g. BaSO4 or TiO2, but these materials have low luminous efficiency, since much of the light is backscattered.
Recently, the so-called scattering beads are used for diffusion applications, which are crosslinked materials whose refractive index differs from that of the matrix. These moulding compositions currently use PMMA-based scattering particles with particle size well above 40 xcexcm. The advantage of these scattering particles is a high level of forward scattering by the mouldings offering cooperation of the scattering particles into the moulding compositions. One way of determining the extent of this preferred forward scattering is to measure transmittance combined with the halved-energy angle on mouldings which include scattering beads.
The smaller the size of the scattering beads, the greater their scattering effect. It stands that the amount of beads used can be reduced if the beads are smaller. This reduction in the amount of beads saves costs and conserves resources. Moulding compositions which contain these smaller bead polymers have excellent mechanical properties, since the reduced amount of scattering beads has less effect on these properties. There is a problem, however, in that the perceived yellowness of the resultant moulding compositions increases markedly if scattering beads having a diameter less than 5 xcexcm are used.
A specific experiment in which a cast PMMA sheet including light-scattering particles is illuminated laterally reveals that the ideal particles have a size in the range from 5 to 20 xcexcm with very narrow size distribution.
Polymer particles whose size is of the order of from 1-10 xcexcm can readily be prepared by precipitation polymerization, using large amounts of organic solvents. However, the handling of the solvents used creates safety and disposal problems. Work-up processes are also difficult. Beads obtained in this way are therefore expensive and cost reasons prevent their use in the application sectors described above.
Polymer beads can be obtained at lower cost by conventional suspension polymerization. However, the size of the resultant particles is generally greater than 40 xcexcm, with an undesirably broad distribution.
EP-A-0 443 609, for example, discloses a suspension process for preparing bead polymers, by using a mixing cell with high shear energy to combine two phases introduced separately (monomers and continuous phase) and then polymerizing the monomers in a conventional reaction vessel. Various auxiliaries are mentioned for stabilizing the dispersion. These include inorganic substances, such as calcium phosphate, and organic compounds, such as cellulose derivatives or polyvinyl alcohol. EP-A-0 443 609 does not describe the use of aluminum compounds.
The examples in EP-A-0 443 609 state that particle sizes in the range from 5 to 10 xcexcm are obtained. However, the monomer mixture was composed of 80% by weight of styrene. If these experiments are repeated using mixtures which include more than 50% by weight of acrylic monomers the particles become much larger. Although the use of known organic stabilizers gives small particles, if these particles are processed into moulding compositions marked yellowing occurs, causing increased reflection, i.e. reduced luminous efficiency.
One object of the present invention is to avoid the problems associated with the conventional compositions.
Another object of the present invention is to provide diffuse-effect moulding compositions, which can be prepared at low cost and in a manner compatible with the environment.
Another object of the present invention is to provide bead polymers which are suitable for incorporation into moulding compositions.
Another object of the present invention is to provide bead polymers having an average size in the range from 5 to 40 xcexcm, without using large amounts of organic solvents that would require removal after the polymerization process.
Another object of the invention is to provide diffuse-effect moulding compositions blended with bead polymer, where the bead polymers are obtainable by processes which can be carried out using commercially available systems.
Another object of the invention is to provide a process which prepares bead polymers suitable for use in moulding compositions and which can be implemented without any major safety risk.
Another object of the present invention is to provide a process for preparing bead polymers which avoids the release or handling of pollutants.
Another object of the present invention is to provide a process which prepares bead polymers wherein the resultant suspension can be worked up by filtration.
Another object of the present invention is to provide a process which prepares bead polymers wherein the resultant suspension can be worked up by filtration under superatmospheric pressure.
Another object of the present invention is to provide a process which prepares bead polymers wherein the resultant suspension can be worked up by filtration under subatmospheric pressure.
Another object of the present invention is to provide diffuse-effect mouldings which have excellent properties and which can be produced at low cost.
Another object of the present invention is to provide mouldings which have high transmittance, little yellowing, and a high halved-energy angle.
Another object of the present invention is to provide mouldings having excellent mechanical properties.
Another object of the present invention is to provide mouldings which have high impact strength and weather resistance.
Another object of the present invention is to provide mouldings which, when exposed to light over a prolonged period, does not show any of the discoloration and particularly the type of discoloration which can occur when the bead polymers degrade.
Another object of the present invention is to provide mouldings which have high scratch resistance.
These and other objects of the present invention have now been achieved by the present invention, the first embodiment of which provides a moulding composition, which includes:
a plurality of bead polymers dispersed in at least one matrix polymer;
wherein the bead polymers:
have an average particle size of 5 to 40 xcexcm, and
have a refractive index nD at 20xc2x0 C. which is different from a refractive index nD at 20xc2x0 C. of said matrix polymer;
and wherein the bead polymers are prepared by a process, which includes:
contacting:
at least one polymerizable mix which includes at least 50% by weight of at least one (meth)acrylate monomer,
at least one aluminum compound, and
an aqueous phase,
to prepare a mixture;
dispersing the mixture at a shear rate xe2x89xa7103 sxe2x88x921 to form a dispersion, wherein the dispersion is stabilized by the aluminum compound; and
polymerizing to produce the bead polymers having an average particle size of 5 to 40 xcexcm.
Another embodiment of the present invention provides an article, which includes the abovementioned composition.
Another embodiment of the present invention provides a method of diffusing light, which includes exposing the abovementioned composition to light.
Another embodiment of the present invention provides a method of making a light-diffusing article, which includes molding the abovementioned composition.